Telomere elongation in immortal human cells without detectable telomerase activity.

Abstract

Immortalization of human cells is often associated with reactivation of telomerase, a ribonucleoprotein enzyme that adds TTAGGG repeats onto telomeres and compensates for their shortening. We examined whether telomerase activation is necessary for immortalization. All normal human fibroblasts tested were negative for telomerase activity. Thirteen out of 13 DNA tumor virus‐transformed cell cultures were also negative in the pre‐crisis (i.e. non‐immortalized) stage. Of 35 immortalized cell lines, 20 had telomerase activity as expected, but 15 had no detectable telomerase. The 15 telomerase‐negative immortalized cell lines all had very long and heterogeneous telomeres of up to 50 kb. Hybrids between telomerase‐negative and telomerase‐positive cells senesced. Two senescent hybrids demonstrated telomerase activity, indicating that activation of telomerase is not sufficient for immortalization. Some hybrid clones subsequently recommenced proliferation and became immortalized either with or without telomerase activity. Those without telomerase activity also had very long and heterogeneous telomeres. Taken together, these data suggest that the presence of lengthened or stabilized telomeres is necessary for immortalization, and that this may be achieved either by the reactivation of telomerase or by a novel and as yet unidentified mechanism.

Targeted therapy of human malignant glioma in a mouse model by 2-5A antisense directed against telomerase RNA

“…In contrast, when IL-2 dependent cells were deprived of IL-2, expression levels of both Bcl-2 and telomerase were reduced. There is further evidence that the mere presence of telomerase may not be essential for immortality (Bryan et al, 1995) since somatic cell hybrids between telomerase positive cells and telomerase-negative cells usually show that senescence is dominant over the malignant phenotype.…”

Telomerase activity in 144 brain tumours

“…Langford et al (1995) found that some anaplastic astrocytomas with malignant potential showed no telomerase activity and suspected an alternative mechanism of maintaining telomere length. Bryan et al (1995) recently described a telomerase-independent mechanism in an immortalized human cell line without detectable telomerase activity. The third reason relates to the sensitivity of our assay.…”

Expression of telomerase in normal and malignant rat hepatic epithelia

“…The initial telomerase extension reaction and the secondary PCR reaction were performed in di erent tubes. A phenolchloroform extraction of the telomerase-extended product was used to eliminate potential inhibitors of the secondary PCR reaction (Bryan et al, 1995). The telomerase extension reaction was performed at room temperature for 30 min.…”

“…Consequently, telomerase expession has been proposed to be required for cellular immortalization . However, other mechanisms of telomere maintenance are also possible (Bryan et al, 1995;Strahl and Blackburn, 1996), such as recombinational events as seen in yeast (Wang and Zakian, 1990) or transposition of retrotransposons as found in Drosophila (Beissmann and Mason, 1992; Levis et al, 1993). Alterations in the regulation of telomerase expression may play a critical role in the immortalization stage of carcinogenesis.…”